The present invention relates to the fixturing of workpieces in general, and more particularly to improvements in fixturing of workpieces in castings which can be used to support workpieces during treatment in machine tools, especially during treatment of portions of turbine guide vanes in grinding machines.
It is often necessary to subject selected portions of freshly forged or cast turbine or compressor blades to a secondary treatment, e.g., in a grinding machine. Such workpieces are often confined in cast fixtures or jigs which are used to clamp the workpieces in the work holder of a machine tool for convenient removal of material from the ends of turbine blades or like workpieces. As a rule, the material of the fixture is a metal which melts at a relatively low temperature and surrounds certain portions of a workpiece, namely those portions which need not be treated by a grinding wheel and/or another material removing tool. For example, turbine blades and similar workpieces can be encapsulated in fixtures which consist of a hardened tin-lead-antimony alloy with a melting point well below 200.degree. C. An advantage of such encapsulation or fixturing of workpieces is that the fixture can be caused to assume a shape which is particularly suitable for convenient manipulation in a grinding machine or in another machine tool, and also that the sensitive parts of a workpiece which need not be treated by material removing tools are shielded while the exposed parts of the workpiece are treated in the selected machine tool.
When the treatment of an encapsulated workpiece is completed, it is necessary to destroy the fixture in order to gain access to the finished workpiece for the purpose of storage, further processing or assembly with similar or other parts into guide vane rings for use in compressors, turbines or the like.
One heretofore known proposal to remove the fixture from the treated workpiece is to heat the fixture to the melting point of its material. A drawback of such proposal is that, as a rule, at least a small amount of molten material of the fixture adheres to the finished workpiece and must be removed in a time-consuming and costly operation. Secondary treatment for the purpose of removing the material which deposits upon a workpiece during melting of the fixture cannot be avoided if the workpiece is a turbine blade or comprises a set of two or more coherent turbine blades because the deposits of metallic material which would remain on the blade or blades could entail rapid or immediate destruction of the corresponding part of a turbine.
In accordance with another prior proposal, an untreated workpiece is coated with a layer of hardenable synthetic plastic material prior to encapsulation in a metallic fixture which is cast around selected portions of the workpiece. This proposal exhibits the drawback that removal of the plastic layer from the workpiece upon destruction of the cast fixture is time-consuming and expensive.
German Auslegeschrift No. 28 22 828 proposes to subject the fixture to the action of a force which suffices to break up the fixture into two or more parts. The apparatus for breaking up the fixture comprises two rams which are provided with prongs and at least one of which is movable relative to the other ram to thereby crush the fixture. The prongs of the rams extend into slots which are provided in the fixture. At least one of the rams is moved at right angles to and toward the other ram so as to break the fixture along a rated break point. The apparatus which is disclosed in the German publication is suitable for breaking up fixtures which surround relatively simple workpieces, such as discrete turbine blades, but is not capable of predictably comminuting or crushing a fixture which serves to confine selected portions of a rather complex workpiece, e.g., a set of turbine blades which are integral with arcuate inner and outer guide vane ring segments. Heretofore known proposals to cast fixtures for such relatively complex workpieces involve the utilization of a mold wherein the compartment or compartments between the blades of a workpiece having two or more spaced-apart blades are filled with the material of the fixture. Therefore, the thus obtained fixture cannot be comminuted or broken up by expanding it in response to penetration of a breaking tool into the compartment or compartments between the blades because such compartment or compartments are filled with hardened material of the fixture. The only reliable mode of breaking up such fixtures is to heat them to the melting point of their material which, however, brings about the aforediscussed drawbacks, particularly the likelihood of deposition of molten material of the fixture on the finished product.
In accordance with another prior proposal, complex workpieces in the form of twin turbine blades or the like are mechanically clamped in conventional jigs which are thereupon manipulated in a machine tool to remove material from exposed portions of the workpieces. Proper retention of a complex workpiece in a jig which is not a casting necessitates extensive experience and carefulness on the part of the attendants and takes up more time than the making of a fixture from molten metallic or other suitable material. The attendants must be very careful to properly clamp the workpiece in a mechanical jig and to thereupon properly mount the jib in the machine tool so as to ensure that certain portions of the workpiece will be adequately concealed and cannot be damaged during removal of material from exposed portions as well as that the workpiece can be subjected to material removing forces of required magnitude so as to terminate the material removing action within a reasonably short interval of time. Moreover, the workpiece must be clamped with a sufficient force to eliminate the likelihood of shifting during removal of material. If the workpiece is not properly mounted in the machine tool, the application of pronounced stresses in the course of the material removing operation is highly likely to entail damage to the workpiece.